Towers for wind turbines have preferably been made of steel by the wind turbine industry for the past 30 years. Development of the industry has been towards larger wind turbines with rotors of increasing diameter. This has required the wind turbine tower to increase in height.
The cost of wind turbine towers has therefore increase over the past years, and especially the logistics of transporting the steel sections comprising the wind turbine towers have proved to be a challenge. Usually the towers have been transported in sections of up to 30 meters. The towers now approach a height of 100 meters and more. Therefore the tower may comprise four or more sections.
Steel reinforced concrete towers, which were introduced in the 1930s, have recently gained favour again for tower heights of more than 80 meters. For concrete towers there are two main options: site-mixed concrete or prefabricated concrete towers.
Site-mixed concrete towers have some disadvantages, for example the difficulty in controlling the quality of the concrete and the logistics in transporting the concrete mixture to the site. Site-mixed concrete towers will not be discussed any further.
Prefabricated concrete towers may be manufactured at a central location. The process of mixing concrete and setting the concrete may be controlled in such a way as to produce concrete elements with relatively low manufacturing tolerances. The material will be uniform, and hence the strength of the prefabricated concrete elements will be uniform.
With prefabricated concrete towers, the transportation disadvantages of the steel towers are overcome. Size of the concrete elements composing the tower is selected such that they allow road or rail transport.
A method of fabricating a prefabricated concrete tower is based on segments prefabricated in a central manufacturing site. The segments are produced using conventional manufacturing techniques. When the sections have set, they are transported to the site and placed on top of each other and bonded together. The towers are provided with longitudinal tubes in the wall of each segment evenly distributed along their circumference for inserting tendons, which are tensioned when the assembly is complete. The concrete tower is pre-stressed such that the tower is in tension during all operational conditions of the wind turbine.
An example of such tower is known from WO 02/01025. The disadvantage of the tower described in WO 02/01025 is that the tendons are not accessible for inspection. This has been solved in EP 1 262 614 which describes a tower construction comprising a foundation, a plurality of annular sections placed on top of each other and with a steel tower placed on top of the concrete part of the tower construction. The tower construction has tendons spanning the length of the tower construction. The steel tower placed on top of the concrete section comprises attachments means for one end of the tendons. The other end of the tendons terminates inside a space in the foundation. The tendons are pre stressed against the concrete foundation by a hydraulic jack and wedged against the concrete by an anchoring element.
Although the system described in EP 1 262 614 may be successfully implemented, the system has some disadvantages.
In a tower of considerable height, for example towers exceeding 80 meters, the prestressing of the tendons will apply very high local forces in the foundation where the anchor elements are abutting the foundation. Therefore, the foundation needs to be oversized in order to be able to take up the forces without cracking or the number of tendons need to increase, which in turn, requires the diameter of the tower to increase to accommodate the increased number of tendons.
It is another disadvantage that the tower segments may be difficult to position along the centreline of the tower such that the tower is straight.